Method and apparatus for discriminating tachycardia events in a medical device

ABSTRACT

A method and medical device for detecting a cardiac event that includes sensing cardiac signals from a plurality of electrodes forming a first sensing vector sensing a first interval of the cardiac signal during a predetermined time period and a second sensing vector simultaneously sensing a second interval of the cardiac signal during the predetermined time period, identifying each of the first interval and the second interval as being one of shockable and not shockable in response to first processing of the first interval and the second interval and in response to second processing of one or both of the first interval and the second interval, the second processing being different from the first processing, and determining whether to deliver therapy for the cardiac event in response to identifying each of the first interval and the second interval as being one of shockable and not shockable in response to both the first processing and the second processing of the first interval and the second interval.

This application is a continuation of U.S. patent application Ser. No.15/801,522, (published as U.S. Publication No. 2018/0064949), filed Nov.2, 2017, which was a continuation of U.S. patent application Ser. No.14/250,040 filed Apr. 10, 2014, now U.S. Pat. No. 9,808,640, the contentof both of which is incorporated herein by reference in their entirety.

TECHNICAL FIELD

The disclosure relates generally to implantable medical devices and, inparticular, to an apparatus and method for discriminating arrhythmiasand delivering a therapy in a medical device.

BACKGROUND

Implantable medical devices are available for treating cardiactachyarrhythmias by delivering anti-tachycardia pacing therapies andelectrical shock therapies for cardioverting or defibrillating theheart. Such a device, commonly known as an implantable cardioverterdefibrillator or “ICD”, senses electrical activity from the heart,determines a patient's heart rate, and classifies the rate according toa number of heart rate zones in order to detect episodes of ventriculartachycardia or fibrillation. Typically a number of rate zones aredefined according to programmable detection interval ranges fordetecting slow ventricular tachycardia, fast ventricular tachycardia andventricular fibrillation. Intervals between sensed R-waves,corresponding to the depolarization of the ventricles, are measured.Sensed R-R intervals falling into defined detection interval ranges arecounted to provide a count of ventricular tachycardia (VT) orventricular fibrillation (VF) intervals, for example. A programmablenumber of intervals to detect (NID) defines the number of tachycardiaintervals occurring consecutively or out of a given number of precedingevent intervals that are required to detect VT or VF.

Tachyarrhythmia detection may begin with detecting a fast ventricularrate, referred to as rate- or interval-based detection. Once VT or VF isdetected based on rate, the morphology of the sensed depolarizationsignals, e.g. wave shape, amplitude or other features, may be used indiscriminating heart rhythms to improve the sensitivity and specificityof tachyarrhythmia detection methods.

A primary goal of a tachycardia detection algorithm is to rapidlyrespond to a potentially malignant rhythm with a therapy that willterminate the arrhythmia with high certainty. Another goal, however, isto avoid excessive use of ICD battery charge, which shortens the life ofthe ICD, e.g. due to delivering unnecessary therapies or therapies at ahigher voltage than needed to terminate a detected tachyarrhythmia.Minimizing the patient's exposure to painful shock therapies is also animportant consideration. Accordingly, a need remains for ICDs thatperform tachycardia discrimination with high specificity and controltherapy delivery to successfully terminate a detected VT requiringtherapy while conserving battery charge and limiting patient exposure todelivered shock therapy by withholding therapy delivery wheneverpossible in situations where the therapy may not be required.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a conceptual diagram of a patient implanted with an exampleextravascular cardiac defibrillation system.

FIG. 2 is an exemplary schematic diagram of electronic circuitry withina hermetically sealed housing of a subcutaneous device according to anembodiment of the present invention.

FIG. 3 is a state diagram of detection of arrhythmias in a medicaldevice according to an embodiment of the present invention.

FIG. 4 is a flowchart of a method for detecting arrhythmias in asubcutaneous device according to an embodiment of the presentdisclosure.

FIG. 5 is a flowchart of a method of determining noise according to anembodiment of the present disclosure.

FIG. 6A is a graphical representation of a determination of whether asignal is corrupted by muscle noise according to an embodiment of thepresent invention.

FIG. 6B is a flowchart of a method of determining whether a signal iscorrupted by muscle noise according to an embodiment of the presentinvention.

FIG. 6C is a flowchart of a method of determining whether a signal iscorrupted by muscle noise according to an embodiment of the presentinvention.

FIG. 7 is a graphical representation of a VF shock zone according to anembodiment of the present invention.

FIGS. 8A and 8B are graphical representations of the determination ofwhether an event is within a shock zone according to an embodiment ofthe present invention.

FIG. 9 is a flowchart of a method for discriminating cardiac eventsaccording to an embodiment of the disclosure.

FIG. 10 is a flowchart of a beat-based analysis during detection ofarrhythmias in a medical device according to an embodiment of thepresent disclosure.

FIG. 11 is a flowchart of a beat-based analysis during detection ofarrhythmias in a medical device according to an embodiment of thepresent disclosure.

DETAILED DESCRIPTION

FIG. 1 is a conceptual diagram of a patient 12 implanted with an exampleextravascular cardiac defibrillation system 10. In the exampleillustrated in FIG. 1 , extravascular cardiac defibrillation system 10is an implanted subcutaneous ICD system. However, the techniques of thisdisclosure may also be utilized with other extravascular implantedcardiac defibrillation systems, such as a cardiac defibrillation systemhaving a lead implanted at least partially in a substernal orsubmuscular location. Additionally, the techniques of this disclosuremay also be utilized with other implantable systems, such as implantablepacing systems, implantable neurostimulation systems, drug deliverysystems or other systems in which leads, catheters or other componentsare implanted at extravascular locations within patient 12. Thisdisclosure, however, is described in the context of an implantableextravascular cardiac defibrillation system for purposes ofillustration.

Extravascular cardiac defibrillation system 10 includes an implantablecardioverter defibrillator (ICD) 14 connected to at least oneimplantable cardiac defibrillation lead 16. ICD 14 of FIG. 1 isimplanted subcutaneously on the left side of patient 12. Defibrillationlead 16, which is connected to ICD 14, extends medially from ICD 14toward sternum 28 and xiphoid process 24 of patient 12. At a locationnear xiphoid process 24, defibrillation lead 16 bends or turns andextends subcutaneously superior, substantially parallel to sternum 28.In the example illustrated in FIG. 1 , defibrillation lead 16 isimplanted such that lead 16 is offset laterally to the left side of thebody of sternum 28 (i.e., towards the left side of patient 12).

Defibrillation lead 16 is placed along sternum 28 such that a therapyvector between defibrillation electrode 18 and a second electrode (suchas a housing or can 25 of ICD 14 or an electrode placed on a secondlead) is substantially across the ventricle of heart 26. The therapyvector may, in one example, be viewed as a line that extends from apoint on the defibrillation electrode 18 to a point on the housing orcan 25 of ICD 14. In another example, defibrillation lead 16 may beplaced along sternum 28 such that a therapy vector betweendefibrillation electrode 18 and the housing or can 25 of ICD 14 (orother electrode) is substantially across an atrium of heart 26. In thiscase, extravascular ICD system 10 may be used to provide atrialtherapies, such as therapies to treat atrial fibrillation.

The embodiment illustrated in FIG. 1 is an example configuration of anextravascular ICD system 10 and should not be considered limiting of thetechniques described herein. For example, although illustrated as beingoffset laterally from the midline of sternum 28 in the example of FIG. 1, defibrillation lead 16 may be implanted such that lead 16 is offset tothe right of sternum 28 or more centrally located over sternum 28.Additionally, defibrillation lead 16 may be implanted such that it isnot substantially parallel to sternum 28, but instead offset fromsternum 28 at an angle (e.g., angled lateral from sternum 28 at eitherthe proximal or distal end). As another example, the distal end ofdefibrillation lead 16 may be positioned near the second or third rib ofpatient 12. However, the distal end of defibrillation lead 16 may bepositioned further superior or inferior depending on the location of ICD14, location of electrodes 18, 20, and 22, or other factors.

Although ICD 14 is illustrated as being implanted near a midaxillaryline of patient 12, ICD 14 may also be implanted at other subcutaneouslocations on patient 12, such as further posterior on the torso towardthe posterior axillary line, further anterior on the torso toward theanterior axillary line, in a pectoral region, or at other locations ofpatient 12. In instances in which ICD 14 is implanted pectorally, lead16 would follow a different path, e.g., across the upper chest area andinferior along sternum 28. When the ICD 14 is implanted in the pectoralregion, the extravascular ICD system may include a second lead includinga defibrillation electrode that extends along the left side of thepatient such that the defibrillation electrode of the second lead islocated along the left side of the patient to function as an anode orcathode of the therapy vector of such an ICD system.

ICD 14 includes a housing or can 25 that forms a hermetic seal thatprotects components within ICD 14. The housing 25 of ICD 14 may beformed of a conductive material, such as titanium or other biocompatibleconductive material or a combination of conductive and non-conductivematerials. In some instances, the housing 25 of ICD 14 functions as anelectrode (referred to as a housing electrode or can electrode) that isused in combination with one of electrodes 18, 20, or 22 to deliver atherapy to heart 26 or to sense electrical activity of heart 26. ICD 14may also include a connector assembly (sometimes referred to as aconnector block or header) that includes electrical feedthroughs throughwhich electrical connections are made between conductors withindefibrillation lead 16 and electronic components included within thehousing. Housing may enclose one or more components, includingprocessors, memories, transmitters, receivers, sensors, sensingcircuitry, therapy circuitry and other appropriate components (oftenreferred to herein as modules).

Defibrillation lead 16 includes a lead body having a proximal end thatincludes a connector configured to connect to ICD 14 and a distal endthat includes one or more electrodes 18, 20, and 22. The lead body ofdefibrillation lead 16 may be formed from a non-conductive material,including silicone, polyurethane, fluoropolymers, mixtures thereof, andother appropriate materials, and shaped to form one or more lumenswithin which the one or more conductors extend. However, the techniquesare not limited to such constructions. Although defibrillation lead 16is illustrated as including three electrodes 18, 20 and 22,defibrillation lead 16 may include more or fewer electrodes.

Defibrillation lead 16 includes one or more elongated electricalconductors (not illustrated) that extend within the lead body from theconnector on the proximal end of defibrillation lead 16 to electrodes18, 20 and 22. In other words, each of the one or more elongatedelectrical conductors contained within the lead body of defibrillationlead 16 may engage with respective ones of electrodes 18, 20 and 22.When the connector at the proximal end of defibrillation lead 16 isconnected to ICD 14, the respective conductors may electrically coupleto circuitry, such as a therapy module or a sensing module, of ICD 14via connections in connector assembly, including associatedfeedthroughs. The electrical conductors transmit therapy from a therapymodule within ICD 14 to one or more of electrodes 18, 20 and 22 andtransmit sensed electrical signals from one or more of electrodes 18, 20and 22 to the sensing module within ICD 14.

ICD 14 may sense electrical activity of heart 26 via one or more sensingvectors that include combinations of electrodes 20 and 22 and thehousing or can 25 of ICD 14. For example, ICD 14 may obtain electricalsignals sensed using a sensing vector between electrodes 20 and 22,obtain electrical signals sensed using a sensing vector betweenelectrode 20 and the conductive housing or can 25 of ICD 14, obtainelectrical signals sensed using a sensing vector between electrode 22and the conductive housing or can 25 of ICD 14, or a combinationthereof. In some instances, ICD 14 may sense cardiac electrical signalsusing a sensing vector that includes defibrillation electrode 18, suchas a sensing vector between defibrillation electrode 18 and one ofelectrodes 20 or 22, or a sensing vector between defibrillationelectrode 18 and the housing or can 25 of ICD 14.

ICD may analyze the sensed electrical signals to detect tachycardia,such as ventricular tachycardia or ventricular fibrillation, and inresponse to detecting tachycardia may generate and deliver an electricaltherapy to heart 26. For example, ICD 14 may deliver one or moredefibrillation shocks via a therapy vector that includes defibrillationelectrode 18 of defibrillation lead 16 and the housing or can 25.Defibrillation electrode 18 may, for example, be an elongated coilelectrode or other type of electrode. In some instances, ICD 14 maydeliver one or more pacing therapies prior to or after delivery of thedefibrillation shock, such as anti-tachycardia pacing (ATP) or postshock pacing. In these instances, ICD 14 may generate and deliver pacingpulses via therapy vectors that include one or both of electrodes 20 and22 and/or the housing or can 25. Electrodes 20 and 22 may comprise ringelectrodes, hemispherical electrodes, coil electrodes, helix electrodes,segmented electrodes, directional electrodes, or other types ofelectrodes, or combination thereof. Electrodes 20 and 22 may be the sametype of electrodes or different types of electrodes, although in theexample of FIG. 1 both electrodes 20 and 22 are illustrated as ringelectrodes.

Defibrillation lead 16 may also include an attachment feature 29 at ortoward the distal end of lead 16. The attachment feature 29 may be aloop, link, or other attachment feature. For example, attachment feature29 may be a loop formed by a suture. As another example, attachmentfeature 29 may be a loop, link, ring of metal, coated metal or apolymer. The attachment feature 29 may be formed into any of a number ofshapes with uniform or varying thickness and varying dimensions.Attachment feature 29 may be integral to the lead or may be added by theuser prior to implantation. Attachment feature 29 may be useful to aidin implantation of lead 16 and/or for securing lead 16 to a desiredimplant location. In some instances, defibrillation lead 16 may includea fixation mechanism in addition to or instead of the attachmentfeature. Although defibrillation lead 16 is illustrated with anattachment feature 29, in other examples lead 16 may not include anattachment feature 29.

Lead 16 may also include a connector at the proximal end of lead 16,such as a DF4 connector, bifurcated connector (e.g., DF-1/IS-1connector), or other type of connector. The connector at the proximalend of lead 16 may include a terminal pin that couples to a port withinthe connector assembly of ICD 14. In some instances, lead 16 may includean attachment feature at the proximal end of lead 16 that may be coupledto an implant tool to aid in implantation of lead 16. The attachmentfeature at the proximal end of the lead may separate from the connectorand may be either integral to the lead or added by the user prior toimplantation.

Defibrillation lead 16 may also include a suture sleeve or otherfixation mechanism (not shown) located proximal to electrode 22 that isconfigured to fixate lead 16 near the xiphoid process or lower sternumlocation. The fixation mechanism (e.g., suture sleeve or othermechanism) may be integral to the lead or may be added by the user priorto implantation.

The example illustrated in FIG. 1 is exemplary in nature and should notbe considered limiting of the techniques described in this disclosure.For instance, extravascular cardiac defibrillation system 10 may includemore than one lead. In one example, extravascular cardiac defibrillationsystem 10 may include a pacing lead in addition to defibrillation lead16.

In the example illustrated in FIG. 1 , defibrillation lead 16 isimplanted subcutaneously, e.g., between the skin and the ribs orsternum. In other instances, defibrillation lead 16 (and/or the optionalpacing lead) may be implanted at other extravascular locations. In oneexample, defibrillation lead 16 may be implanted at least partially in asubsternal location. In such a configuration, at least a portion ofdefibrillation lead 16 may be placed under or below the sternum in themediastinum and, more particularly, in the anterior mediastinum. Theanterior mediastinum is bounded laterally by pleurae, posteriorly bypericardium, and anteriorly by sternum 28. Defibrillation lead 16 may beat least partially implanted in other extra-pericardial locations, i.e.,locations in the region around, but not in direct contact with, theouter surface of heart 26. These other extra-pericardial locations mayinclude in the mediastinum but offset from sternum 28, in the superiormediastinum, in the middle mediastinum, in the posterior mediastinum, inthe sub-xiphoid or inferior xiphoid area, near the apex of the heart, orother location not in direct contact with heart 26 and not subcutaneous.In still further instances, the lead may be implanted at a pericardialor epicardial location outside of the heart 26.

FIG. 2 is an exemplary schematic diagram of electronic circuitry withina hermetically sealed housing of a subcutaneous device according to anembodiment of the present invention. As illustrated in FIG. 2 ,subcutaneous device 14 includes a low voltage battery 153 coupled to apower supply (not shown) that supplies power to the circuitry of thesubcutaneous device 14 and the pacing output capacitors to supply pacingenergy in a manner well known in the art. The low voltage battery 153may be formed of one or two conventional LiCF_(x), LiMnO₂ or LiI₂ cells,for example. The subcutaneous device 14 also includes a high voltagebattery 112 that may be formed of one or two conventional LiSVO orLiMnO₂ cells. Although two both low voltage battery and a high voltagebattery are shown in FIG. 2 , according to an embodiment of the presentinvention, the device 14 could utilize a single battery for both highand low voltage uses.

Further referring to FIG. 2 , subcutaneous device 14 functions arecontrolled by means of software, firmware and hardware thatcooperatively monitor the ECG signal, determine when acardioversion-defibrillation shock or pacing is necessary, and deliverprescribed cardioversion-defibrillation and pacing therapies. Thesubcutaneous device 14 may incorporate circuitry set forth in commonlyassigned U.S. Pat. No. 5,163,427 “Apparatus for Delivering Single andMultiple Cardioversion and Defibrillation Pulses” to Keimel and5,188,105 “Apparatus and Method for Treating a Tachyarrhythmia” toKeimel for selectively delivering single phase, simultaneous biphasicand sequential biphasic cardioversion-defibrillation shocks typicallyemploying ICD IPG housing electrodes 28 coupled to the COMMON output 123of high voltage output circuit 140 and cardioversion-defibrillationelectrode 24 disposed posterially and subcutaneously and coupled to theHVI output 113 of the high voltage output circuit 140.

The cardioversion-defibrillation shock energy and capacitor chargevoltages can be intermediate to those supplied by ICDs having at leastone cardioversion-defibrillation electrode in contact with the heart andmost AEDs having cardioversion-defibrillation electrodes in contact withthe skin. The typical maximum voltage necessary for ICDs using mostbiphasic waveforms is approximately 750 Volts with an associated maximumenergy of approximately 40 Joules. The typical maximum voltage necessaryfor AEDs is approximately 2000-5000 Volts with an associated maximumenergy of approximately 200-360 Joules depending upon the model andwaveform used. The subcutaneous device 14 of the present invention usesmaximum voltages in the range of about 300 to approximately 1000 Voltsand is associated with energies of approximately 25 to 150 joules ormore. The total high voltage capacitance could range from about 50 toabout 300 microfarads. Such cardioversion-defibrillation shocks are onlydelivered when a malignant tachyarrhythmia, e.g., ventricularfibrillation is detected through processing of the far field cardiac ECGemploying the detection algorithms as described herein below.

In FIG. 2 , sense amp 190 in conjunction with pacer/device timingcircuit 178 processes the far field ECG sense signal that is developedacross a particular ECG sense vector defined by a selected pair of thesubcutaneous electrodes 18, 20, 22 and the can or housing 25 of thedevice 14, or, optionally, a virtual signal (i.e., a mathematicalcombination of two vectors) if selected. The selection of the sensingelectrode pair is made through the switch matrix/MUX 191 in a manner toprovide the most reliable sensing of the ECG signal of interest, whichwould be the R wave for patients who are believed to be at risk ofventricular fibrillation leading to sudden death. The far field ECGsignals are passed through the switch matrix/MUX 191 to the input of thesense amplifier 190 that, in conjunction with pacer/device timingcircuit 178, evaluates the sensed EGM. Bradycardia, or asystole, istypically determined by an escape interval timer within the pacer timingcircuit 178 and/or the control circuit 144. Pace Trigger signals areapplied to the pacing pulse generator 192 generating pacing stimulationwhen the interval between successive R-waves exceeds the escapeinterval. Bradycardia pacing is often temporarily provided to maintaincardiac output after delivery of a cardioversion-defibrillation shockthat may cause the heart to slowly beat as it recovers back to normalfunction. Sensing subcutaneous far field signals in the presence ofnoise may be aided by the use of appropriate denial and extensibleaccommodation periods as described in U.S. Pat. No. 6,236,882 “NoiseRejection for Monitoring ECGs” to Lee, et al and incorporated herein byreference in its' entirety.

Detection of a malignant tachyarrhythmia is determined in the Controlcircuit 144 as a function of the intervals between R-wave sense eventsignals that are output from the pacer/device timing 178 and senseamplifier circuit 190 to the timing and control circuit 144. It shouldbe noted that the present invention utilizes not only interval basedsignal analysis method but also supplemental sensors and morphologyprocessing method and apparatus as described herein below.

Supplemental sensors such as tissue color, tissue oxygenation,respiration, patient activity and the like may be used to contribute tothe decision to apply or withhold a defibrillation therapy as describedgenerally in U.S. Pat. No. 5,464,434 “Medical Interventional DeviceResponsive to Sudden Hemodynamic Change” to Alt and incorporated hereinby reference in its entirety. Sensor processing block 194 providessensor data to microprocessor 142 via data bus 146. Specifically,patient activity and/or posture may be determined by the apparatus andmethod as described in U.S. Pat. No. 5,593,431 “Medical ServiceEmploying Multiple DC Accelerometers for Patient Activity and PostureSensing and Method” to Sheldon and incorporated herein by reference inits entirety. Patient respiration may be determined by the apparatus andmethod as described in U.S. Pat. No. 4,567,892 “Implantable CardiacPacemaker” to Plicchi, et al and incorporated herein by reference in itsentirety. Patient tissue oxygenation or tissue color may be determinedby the sensor apparatus and method as described in U.S. Pat. No.5,176,137 to Erickson, et al and incorporated herein by reference in itsentirety. The oxygen sensor of the '137 patent may be located in thesubcutaneous device pocket or, alternatively, located on the lead 18 toenable the sensing of contacting or near-contacting tissue oxygenationor color.

Certain steps in the performance of the detection algorithm criteria arecooperatively performed in microcomputer 142, including microprocessor,RAM and ROM, associated circuitry, and stored detection criteria thatmay be programmed into RAM via a telemetry interface (not shown)conventional in the art. Data and commands are exchanged betweenmicrocomputer 142 and timing and control circuit 144, pacertiming/amplifier circuit 178, and high voltage output circuit 140 via abi-directional data/control bus 146. The pacer timing/amplifier circuit178 and the control circuit 144 are clocked at a slow clock rate. Themicrocomputer 142 is normally asleep, but is awakened and operated by afast clock by interrupts developed by each R-wave sense event, onreceipt of a downlink telemetry programming instruction or upon deliveryof cardiac pacing pulses to perform any necessary mathematicalcalculations, to perform tachycardia and fibrillation detectionprocedures, and to update the time intervals monitored and controlled bythe timers in pacer/device timing circuitry 178.

When a malignant tachycardia is detected, high voltage capacitors 156,158, 160, and 162 are charged to a pre-programmed voltage level by ahigh-voltage charging circuit 164. It is generally consideredinefficient to maintain a constant charge on the high voltage outputcapacitors 156, 158, 160, 162. Instead, charging is initiated whencontrol circuit 144 issues a high voltage charge command HVCHG deliveredon line 145 to high voltage charge circuit 164 and charging iscontrolled by means of bi-directional control/data bus 166 and afeedback signal VCAP from the HV output circuit 140. High voltage outputcapacitors 156, 158, 160 and 162 may be of film, aluminum electrolyticor wet tantalum construction.

The negative terminal of high voltage battery 112 is directly coupled tosystem ground. Switch circuit 114 is normally open so that the positiveterminal of high voltage battery 112 is disconnected from the positivepower input of the high voltage charge circuit 164. The high voltagecharge command HVCHG is also conducted via conductor 149 to the controlinput of switch circuit 114, and switch circuit 114 closes in responseto connect positive high voltage battery voltage EXT B+ to the positivepower input of high voltage charge circuit 164. Switch circuit 114 maybe, for example, a field effect transistor (FET) with itssource-to-drain path interrupting the EXT B+ conductor 118 and its gatereceiving the HVCHG signal on conductor 145. High voltage charge circuit164 is thereby rendered ready to begin charging the high voltage outputcapacitors 156, 158, 160, and 162 with charging current from highvoltage battery 112.

High voltage output capacitors 156, 158, 160, and 162 may be charged tovery high voltages, e.g., 300-1000V, to be discharged through the bodyand heart between the electrode pair of subcutaneouscardioversion-defibrillation electrodes 113 and 123. The details of thevoltage charging circuitry are also not deemed to be critical withregard to practicing the present invention; one high voltage chargingcircuit believed to be suitable for the purposes of the presentinvention is disclosed. High voltage capacitors 156, 158, 160 and 162may be charged, for example, by high voltage charge circuit 164 and ahigh frequency, high-voltage transformer 168 as described in detail incommonly assigned U.S. Pat. No. 4,548,209 “Energy Converter forImplantable Cardioverter” to Wielders, et al. Proper charging polaritiesare maintained by diodes 170, 172, 174 and 176 interconnecting theoutput windings of high-voltage transformer 168 and the capacitors 156,158, 160, and 162. As noted above, the state of capacitor charge ismonitored by circuitry within the high voltage output circuit 140 thatprovides a VCAP, feedback signal indicative of the voltage to the timingand control circuit 144. Timing and control circuit 144 terminates thehigh voltage charge command HVCHG when the VCAP signal matches theprogrammed capacitor output voltage, i.e., thecardioversion-defibrillation peak shock voltage.

Control circuit 144 then develops first and second control signalsNPULSE 1 and NPULSE 2, respectively, that are applied to the highvoltage output circuit 140 for triggering the delivery of cardiovertingor defibrillating shocks. In particular, the NPULSE 1 signal triggersdischarge of the first capacitor bank, comprising capacitors 156 and158. The NPULSE 2 signal triggers discharge of the first capacitor bankand a second capacitor bank, comprising capacitors 160 and 162. It ispossible to select between a plurality of output pulse regimes simply bymodifying the number and time order of assertion of the NPULSE 1 andNPULSE 2 signals. The NPULSE 1 signals and NPULSE 2 signals may beprovided sequentially, simultaneously or individually. In this way,control circuitry 144 serves to control operation of the high voltageoutput stage 140, which delivers high energycardioversion-defibrillation shocks between the pair of thecardioversion-defibrillation electrodes 18 and 25 coupled to the HV-1and COMMON output as shown in FIG. 2 .

Thus, subcutaneous device 14 monitors the patient's cardiac status andinitiates the delivery of a cardioversion-defibrillation shock throughthe cardioversion-defibrillation electrodes 18 and 25 in response todetection of a tachyarrhythmia requiring cardioversion-defibrillation.The high HVCHG signal causes the high voltage battery 112 to beconnected through the switch circuit 114 with the high voltage chargecircuit 164 and the charging of output capacitors 156, 158, 160, and 162to commence. Charging continues until the programmed charge voltage isreflected by the VCAP signal, at which point control and timing circuit144 sets the HVCHG signal low terminating charging and opening switchcircuit 114. The subcutaneous device 14 can be programmed to attempt todeliver cardioversion shocks to the heart in the manners described abovein timed synchrony with a detected R-wave or can be programmed orfabricated to deliver defibrillation shocks to the heart in the mannersdescribed above without attempting to synchronize the delivery to adetected R-wave. Episode data related to the detection of thetachyarrhythmia and delivery of the cardioversion-defibrillation shockcan be stored in RAM for uplink telemetry transmission to an externalprogrammer as is well known in the art to facilitate in diagnosis of thepatient's cardiac state. A patient receiving the device 14 on aprophylactic basis would be instructed to report each such episode tothe attending physician for further evaluation of the patient'scondition and assessment for the need for implantation of a moresophisticated ICD.

Subcutaneous device 14 desirably includes telemetry circuit (not shownin FIG. 2 ), so that it is capable of being programmed by means ofexternal programmer 20 via a 2-way telemetry link (not shown). Uplinktelemetry allows device status and diagnostic/event data to be sent toexternal programmer 20 for review by the patient's physician. Downlinktelemetry allows the external programmer via physician control to allowthe programming of device function and the optimization of the detectionand therapy for a specific patient. Programmers and telemetry systemssuitable for use in the practice of the present invention have been wellknown for many years. Known programmers typically communicate with animplanted device via a bi-directional radio-frequency telemetry link, sothat the programmer can transmit control commands and operationalparameter values to be received by the implanted device, so that theimplanted device can communicate diagnostic and operational data to theprogrammer. Programmers believed to be suitable for the purposes ofpracticing the present invention include the Models 9790 and CareLink®programmers, commercially available from Medtronic, Inc., Minneapolis,Minn.

Various telemetry systems for providing the necessary communicationschannels between an external programming unit and an implanted devicehave been developed and are well known in the art. Telemetry systemsbelieved to be suitable for the purposes of practicing the presentinvention are disclosed, for example, in the following U.S. Patents:U.S. Pat. No. 5,127,404 to Wyborny et al. entitled “Telemetry Format forImplanted Medical Device”; U.S. Pat. No. 4,374,382 to Markowitz entitled“Marker Channel Telemetry System for a Medical Device”; and U.S. Pat.No. 4,556,063 to Thompson et al. entitled “Telemetry System for aMedical Device”. The Wyborny et al. '404, Markowitz '382, and Thompsonet al. '063 patents are commonly assigned to the assignee of the presentinvention, and are each hereby incorporated by reference herein in theirrespective entireties.

According to an embodiment of the present invention, in order toautomatically select the preferred ECG vector set, it is necessary tohave an index of merit upon which to rate the quality of the signal.“Quality” is defined as the signal's ability to provide accurate heartrate estimation and accurate morphological waveform separation betweenthe patient's usual sinus rhythm and the patient's ventriculartachyarrhythmia.

Appropriate indices may include R-wave amplitude, R-wave peak amplitudeto waveform amplitude between R-waves (i.e., signal to noise ratio), lowslope content, relative high versus low frequency power, mean frequencyestimation, probability density function, or some combination of thesemetrics.

Automatic vector selection might be done at implantation or periodically(daily, weekly, monthly) or both. At implant, automatic vector selectionmay be initiated as part of an automatic device turn-on procedure thatperforms such activities as measure lead impedances and batteryvoltages. The device turn-on procedure may be initiated by theimplanting physician (e.g., by pressing a programmer button) or,alternatively, may be initiated automatically upon automatic detectionof device/lead implantation. The turn-on procedure may also use theautomatic vector selection criteria to determine if ECG vector qualityis adequate for the current patient and for the device and leadposition, prior to suturing the subcutaneous device 14 device in placeand closing the incision. Such an ECG quality indicator would allow theimplanting physician to maneuver the device to a new location ororientation to improve the quality of the ECG signals as required. Thepreferred ECG vector or vectors may also be selected at implant as partof the device turn-on procedure. The preferred vectors might be thosevectors with the indices that maximize rate estimation and detectionaccuracy. There may also be an a priori set of vectors that arepreferred by the physician, and as long as those vectors exceed someminimum threshold, or are only slightly worse than some other moredesirable vectors, the a priori preferred vectors are chosen. Certainvectors may be considered nearly identical such that they are not testedunless the a priori selected vector index falls below some predeterminedthreshold.

Depending upon metric power consumption and power requirements of thedevice, the ECG signal quality metric may be measured on the range ofvectors (or alternatively, a subset) as often as desired. Data may begathered, for example, on a minute, hourly, daily, weekly or monthlybasis. More frequent measurements (e.g., every minute) may be averagedover time and used to select vectors based upon susceptibility ofvectors to occasional noise, motion noise, or EMI, for example.

Alternatively, the subcutaneous device 14 may have an indicator/sensorof patient activity (piezo-resistive, accelerometer, impedance, or thelike) and delay automatic vector measurement during periods of moderateor high patient activity to periods of minimal to no activity. Onerepresentative scenario may include testing/evaluating ECG vectors oncedaily or weekly while the patient has been determined to be asleep(using an internal clock (e.g., 2:00 am) or, alternatively, infer sleepby determining the patient's position (via a 2- or 3-axis accelerometer)and a lack of activity).

If infrequent automatic, periodic measurements are made, it may also bedesirable to measure noise (e.g., muscle, motion, EMI, etc.) in thesignal and postpone the vector selection measurement when the noise hassubsided.

Subcutaneous device 14 may optionally have an indicator of the patient'sposture (via a 2- or 3-axis accelerometer). This sensor may be used toensure that the differences in ECG quality are not simply a result ofchanging posture/position. The sensor may be used to gather data in anumber of postures so that ECG quality may be averaged over thesepostures or, alternatively, selected for a preferred posture.

In the preferred embodiment, vector quality metric calculations wouldoccur a number of times over approximately 1 minute, once per day, foreach vector. These values would be averaged for each vector over thecourse of one week. Averaging may consist of a moving average orrecursive average depending on time weighting and memory considerations.In this example, the preferred vector(s) would be selected once perweek.

FIG. 3 is a state diagram of detection of arrhythmias in a medicaldevice according to an embodiment of the present invention. Asillustrated in FIG. 3 , during normal operation, the device 14 is in anot concerned state 302, during which R-wave intervals are beingevaluated to identify periods of rapid rates and/or the presence ofasystole. Upon detection of short R-wave intervals simultaneously in twoseparate ECG sensing vectors, indicative of an event that, if confirmed,may require the delivery of therapy, the device 14 transitions from thenot concerned state 302 to a concerned state 304. In the concerned state304 the device 14 evaluates a predetermined window of ECG signals todetermine the likelihood that the signal is corrupted with noise and todiscriminate rhythms requiring shock therapy from those that do notrequire shock therapy, using a combination of R-wave intervals and ECGsignal morphology information.

If a rhythm requiring shock therapy continues to be detected while inthe concerned state 304, the device 14 transitions from the concernedstate 304 to an armed state 306. If a rhythm requiring shock therapy isno longer detected while the device is in the concerned state 304 andthe R-wave intervals are determined to no longer be short, the device 14returns to the not concerned state 302. However, if a rhythm requiringshock therapy is no longer detected while the device is in the concernedstate 304, but the R-wave intervals continue to be detected as beingshort, processing continues in the concerned state 304.

In the armed state 306, the device 14 charges the high voltage shockingcapacitors and continues to monitor R-wave intervals and ECG signalmorphology for spontaneous termination. If spontaneous termination ofthe rhythm requiring shock therapy occurs, the device 14 returns to thenot concerned state 302. If the rhythm requiring shock therapy is stilldetermined to be occurring once the charging of the capacitors iscompleted, the device 14 transitions from the armed state 306 to a shockstate 308. In the shock state 308, the device 14 delivers a shock andreturns to the armed state 306 to evaluate the success of the therapydelivered.

The transitioning between the not concerned state 302, the concernedstate 304, the armed state 306 and the shock state 308 may be performedas described in detail in U.S. Pat. No. 7,894,894 to Stadler et al.,incorporated herein by reference in it's entirety.

FIG. 4 is a flowchart of a method for detecting arrhythmias in asubcutaneous device according to an embodiment of the presentdisclosure. As illustrated in FIG. 4 , device 14 continuously evaluatesthe two channels ECG1 and ECG2 associated with two predeterminedelectrode vectors to determine when sensed events occur. For example,the electrode vectors for the two channels ECG1 and ECG2 may include afirst vector (ECG1) selected between electrode 20 positioned on lead 16and the housing or can 25 of ICD 14, while the other electrode vector(ECG 2) is a vertical electrode vector between electrode 20 andelectrode 22 positioned along the lead 16. However, the two sensingchannels may in any combination of possible vectors, including thoseformed by the electrodes shown in FIG. 2 , or other additionalelectrodes (not shown) that may be included along the lead or positionedalong the housing of ICD 14.

According to an embodiment of the present application, for example, thedevice 14 determines whether to transition from the not concerned state302 to the concerned state 304 by determining a heart rate estimate inresponse to the sensing of R-waves, as described in U.S. Pat. No.7,894,894 to Stadler et al., incorporated herein by reference in it'sentirety.

Upon transition from the not concerned state to the concerned state,Block 305, a most recent window of ECG data from both channels ECG1 andECG2 are utilized, such as three seconds, for example, so thatprocessing is triggered in the concerned state 304 by a three-secondtimeout, rather than by the sensing of an R-wave, which is utilized whenin the not concerned state 302. It is understood that while theprocessing is described as being triggered over a three second period,other times periods for the processing time utilized when in theconcerned state 304 may be chosen, but should preferably be within arange of 0.5 to 10 seconds. As a result, although sensing of individualR-waves continues to occur in both channels ECG1 and ECG2 when in theconcerned state 304, and the buffer of 12 R-R intervals continues to beupdated, the opportunities for changing from the concerned state 304 toanother state and the estimates of heart rate only occur once thethree-second timer expires. Upon initial entry to the concerned state304, it is advantageous to process the most recent three-seconds of ECGdata, i.e., ECG data for the three seconds leading up to the transitionto the concerned state 304. This requires a continuous circularbuffering of the most recent three seconds of ECG data even while in thenot concerned state 302.

While in the concerned state 304, the present invention determines howsinusoidal and how noisy the signals are in order to determine thelikelihood that a ventricular fibrillation (VF) or fast ventriculartachycardia (VT) event is taking place, since the more sinusoidal andlow noise the signal is, the more likely a VT/VF event is taking place.As illustrated in FIG. 4 , once the device transitions from the notconcerned state 302 to the concerned state 304, Block 305, a buffer foreach of the two channels ECG 1 and ECG2 for storing classifications of3-second segments of data as “shockable” or “non-shockable” is cleared.Processing of signals of the two channels ECG1 and ECG2 while in theconcerned state 304 is then triggered by the three second time period,rather than by the sensing of an R-wave utilized during the notconcerned state 302.

Once the three second time interval has expired, YES in Block 341,morphology characteristics of the signal during the three second timeinterval for each channel are utilized to determine whether the signalsare likely corrupted by noise artifacts and to characterize themorphology of the signal as “shockable” or “not shockable”. For example,using the signals associated with the three second time interval, adetermination is made for each channel ECG1 and ECG 2 as to whether thechannel is likely corrupted by noise, Block 342, and a determination isthen made as to whether both channels ECG1 and ECG2 are corrupted bynoise, Block 344.

FIG. 5 is a flowchart of a method of determining noise according to anembodiment of the present disclosure. As illustrated in FIG. 5 , thedetermination as to whether the signal associated with each of thechannels ECG1 and ECG2 is likely corrupted by noise, Block 342 of FIG. 4, includes multiple sequential noise tests that are performed on eachchannel ECG and ECG2. During a first noise test, for example, adetermination is made as to whether a metric of signal energy content ofthe signal for the channel is within predetermined limits, Block 380.For example, the amplitude of each sample associated with the threesecond window is determined, resulting in N sample amplitudes, fromwhich a mean rectified amplitude is calculated as the ratio of the sumof the rectified sample amplitudes to the total number of sampleamplitudes N for the segment. If the sampling rate is 256 samples persecond, for example, the total number of sample amplitudes N for thethree-second segment would be N=768 samples.

Once the mean rectified amplitude is calculated, a determination is madeas to whether the mean rectified amplitude is between an upper averageamplitude limit and a lower average amplitude limit, the lower averageamplitude limit being associated with asystole episodes without artifactand the upper average amplitude limit being associated with a valuegreater than what would be associated with ventricular tachycardia andventricular fibrillation events. According to an embodiment of thepresent invention, the upper average amplitude limit is set as 1.5 mV,and the lower average amplitude limit is set as 0.013 mV. While themetric of signal energy content is described above as the mean rectifiedamplitude, it is understood that other signal of energy contents couldbe utilized.

If the determined mean rectified amplitude is not between the upperaverage amplitude limit and the lower average amplitude limit, the threesecond segment for that channel is identified as being likely corruptedwith noise, Block 386, and no further noise tests are initiated for thatchannel's segment.

If the determined mean rectified amplitude is located between the upperaverage amplitude limit and the lower average amplitude limit, a noiseto signal ratio is calculated and a determination is made as to whetherthe noise to signal ratio is less than a predetermined noise to signalthreshold, Block 382. For example, the amplitude of each sampleassociated with the three second window is determined, resulting in Nraw sample amplitudes. The raw signal is lowpass filtered, resulting inL lowpass sample amplitudes. The raw mean rectified amplitude isdetermined as the average of the absolute values of the raw sampleamplitudes. The lowpass mean rectified amplitude is determined as theaverage of the absolute values of the lowpass sample amplitudes. Next, ahighpass mean rectified amplitude is then calculated as the differencebetween the raw mean rectified amplitude and the lowpass mean rectifiedamplitude. The noise to signal ratio is then determined as the ratio ofthe highpass mean rectified amplitude to the lowpass mean rectifiedamplitude. If the noise to signal ratio is greater than a predeterminedthreshold, such as 0.0703, for example, the three second segment forthat channel is identified as being likely corrupted with noise, Block386, and no further noise tests are initiated for the segment.

If the noise to signal ratio is less than or equal to the predeterminedthreshold, a determination is made as to whether the signal is corruptedby muscle noise, Block 384. According to an embodiment of the presentinvention, the determination as to whether the signal is corrupted bymuscle noise is made by determining whether the signal includes apredetermined number of signal inflections indicative of the likelihoodof the signal being corrupted by muscle noise, using a muscle noisepulse count that is calculated to quantify the number of signalinflections in the three second interval for each channel ECG1 and ECG2.The presence of a significant number of inflections is likely indicativeof muscle noise.

FIG. 6A is a graphical representation of a determination of whether asignal is corrupted by muscle noise according to an embodiment of thepresent invention. FIG. 6B is a flowchart of a method of determiningwhether a signal is corrupted by muscle noise according to an embodimentof the present invention. For example, as illustrated in FIGS. 6A and6B, in order to determine a muscle noise count for the three secondinterval, the raw signal 420 is applied to a first order derivativefilter to obtain a derivative signal 422, and all of the zero-crossings424 in the derivative signal 422 are located, Block 460. A data paircorresponding to the data points immediately prior to and subsequent tothe zero crossings 424, points 426 and 428 respectively, for eachcrossing is obtained. The value of the data point in each data pair withsmaller absolute value is zeroed in order to allow a clear demarcationof each pulse when a rectified signal 430 is derived from the derivativesignal 422 with zeroed zero-crossing points 432.

A pulse amplitude threshold Td, for determining whether the identifiedinflection is of a significant amplitude to be identified as beingassociated with muscle noise, is determined, Block 462, by dividing therectified signal from the three second segment into equal sub-segments434, estimating a local maximum amplitude 436-442 for each of thesub-segments 434, and determining whether the local amplitudes 436-442are less than a portion of the maximum amplitude, which is maximumamplitude 440 in the example of FIG. 6A, for the whole three secondsegment. If the local maximum amplitude is less than the portion of themaximum amplitude for the whole three second segment, the local maximumamplitude is replaced by the maximum for the whole three second segmentfor the sub-segment corresponding to that local maximum amplitude.

It is understood that while only two or less zero-crossing points areshown as being located within the sub-segments in the illustration ofFIG. 6A for the sake of simplicity, in fact each of the sub-segments434, which have a length of approximately 750 milliseconds, will containmany inflections, such as every 25 milliseconds, for example.

According to an embodiment of the present invention, the three secondsegment is divided into four sub-segments and the local maximumamplitudes are replaced by the maximum amplitude for the whole segmentif the local maximum amplitude is less than one fifth of the maximumamplitude for the whole segment. Once the determination of whether toreplace the local maximum amplitudes for each of the sub-segments withthe maximum amplitude for the whole segment is completed, the pulseamplitude threshold Td for the segment is set equal to a predeterminedfraction of the mean of the local maximum amplitudes for each of thesub-segments. According to an embodiment of the present invention, thepulse amplitude threshold Td for the three second segment is set equalto one sixth of the mean of the local maximum amplitudes 436-440.

Once the pulse amplitude threshold Td has been determined, theinflections associated with the signal for the three second segment isclassified as being of significant level to be likely indicative ofnoise by determining whether the pulse amplitude threshold Td is lessthan a pulse threshold, Block 464. According to an embodiment of thepresent invention, the pulse threshold is set as 1 microvolt. If thepulse amplitude threshold Td is less than the pulse threshold, thesignal strength is too small for a determination of muscle noise, andtherefore the signal is determined to be not likely corrupted by noiseand therefore the channel is determined to be not noise corrupted, Block466.

If the pulse amplitude threshold Td is greater than or equal to thepulse threshold, the three second segment is divided into twelvesub-segments of 250 ms window length, the number of muscle noise pulsesin each sub-segment is counted, and both the sub-segment having themaximum number of muscle noise pulses and the number of sub-segmentshaving 6 or more muscle noise pulses that are greater than apredetermined minimum threshold is determined. Muscle noise isdetermined to be present in the signal if either the maximum number ofmuscle noise pulses in a single sub-segment is greater than a noisepulse number threshold or the number of sub-segments of the twelvesub-segments having 6 or more muscle noise pulses greater than theminimum threshold is greater than or equal to a sub-segment pulse countthreshold. According to an embodiment of the present invention, thenoise pulse number threshold is set equal to eight and the sub-segmentpulse count threshold is set equal to three.

For example, if the pulse amplitude threshold Td is greater than orequal to the pulse threshold, No in Block 464, the maximum number ofmuscle noise counts in a single sub-segment is determined, Block 468. Ifthe maximum number of muscle noise counts is greater than the noisepulse number threshold, Yes in Block 470, the channel is determined tobe noise corrupted, Block 472. If the maximum number of muscle noisecounts for the channel is less than or equal to the noise pulse numberthreshold, No in Block 470, the number of sub-segments of the twelvesub-segments having 6 or more muscle noise pulses greater than theminimum threshold is determined, Block 474, and if the number is greaterthan or equal to a sub-segment pulse count threshold, Yes in Block 476,the channel is determined to be noise corrupted, Block 472. If thenumber is less than the sub-segment pulse count threshold, No in Block476, the channel is determined not to be noise corrupted, Block 466.

FIG. 6C is a flowchart of a method of determining whether a signal iscorrupted by muscle noise according to an embodiment of the presentinvention. Since muscle noise can be present during an episode ofventricular tachycardia, the width of the overall signal pulse waveformis determined in order to distinguish between signals that aredetermined likely to be purely noise related and signals that are bothshockable events and determined to include noise. Therefore, asillustrated in FIG. 6C, according to an embodiment of the presentinvention, once muscle noise is determined to be present as a result ofthe muscle noise pulse count being satisfied, No in Block 470 and Yes inBlock 476, a determination is made as to whether the signal is bothnoise corrupted and shockable, Block 480.

According to an embodiment of the present invention, the determinationin Block 480 as to whether the signal is both noisy and shockable ismade, for example, by dividing the rectified signal, having 768 datapoints, into four sub-segments and determining a maximum amplitude foreach of the four sub-segments by determining whether a maximum amplitudefor the sub-segment is less than a portion of the maximum amplitude forthe entire rectified signal in the three second segment. For example, adetermination is made for each sub-segment as to whether the maximumamplitude for the sub-segment is less than one fourth of the maximumamplitude for the entire rectified signal. If less than a portion of themaximum amplitude for the entire rectified signal in the three secondsegment, the maximum amplitude for the sub-segment is set equal to themaximum amplitude for the entire rectified signal.

A mean rectified amplitude for each of the sub-segments is determined bydividing the sum of the rectified amplitudes for the sub-segment by thenumber of samples in the sub-segment, i.e., 768÷4. Then the normalizedmean rectified amplitude for each sub-segment is determined by dividingthe mean rectified amplitude for each of the sub-segments by the peakamplitude for the sub-segment. The normalized mean rectified amplitudefor the three second segment is then determined as the sum of thenormalized mean rectified amplitudes for each sub-segment divided by thenumber of sub-segments, i.e., four.

Therefore, once muscle noise is suspected as a result of thedetermination of the muscle noise pulse count, the determination ofBlock 480 based on whether the normalized mean rectified amplitude forthe three second segment is greater than a predetermined threshold foridentifying signals that, despite being indicative of a likelihood ofbeing associated with noise, nevertheless are associated with ashockable event. For example, according to an embodiment of the presentinvention, a determination is made as to whether the normalized meanrectified amplitude for the three second segment is greater than 18microvolts. If the normalized mean rectified amplitude for the threesecond segment is less than or equal to the predetermined threshold, thechannel is likely corrupted by muscle noise and not shockable, No inBlock 480, and is therefore identified as being corrupted by noise,Block 472. If the normalized mean rectified amplitude for the threesecond segment is greater than the predetermined threshold, the channelis determined to be likely corrupted by muscle noise and shockable, Yesin Block 480, and is therefore identified as not to be likely corruptedby muscle noise, Block 478.

Returning to FIG. 5 , when the signal is determined to be not likelycorrupted by muscle noise, a determination is made as to whether themean frequency of the signal associated with the channel is less than apredetermined mean frequency threshold, Block 388, such as 11 Hz forexample. The mean frequency of the signal during the 3 second segmentfor each channel ECG 1 and ECG2 is generated, for example, bycalculating the ratio of the mean absolute amplitude of the firstderivative of the 3 second segment to the mean absolute amplitude of the3 second segment, multiplied by a constant scaling factor. If the meanfrequency is determined to be greater than or equal to the predeterminedmean frequency threshold, No in Block 388, the three second segment forthat channel is identified as being likely corrupted with noise, Block386. If the mean frequency is determined to be less than thepredetermined mean frequency threshold, Yes in Block 388, the threesecond segment for that channel is identified as being not noisecorrupted, Block 390.

According to an embodiment of the present invention, since the meanspectral frequency tends to be low for true ventricular fibrillationevents, moderate for organized rhythms such as sinus rhythm andsupraventricular tachycardia, for example, and high during asystole andnoise, the determination in Block 388 includes determining whether themean frequency is less than a predetermined upper mean frequencythreshold, such as 11 Hz (i.e., mean period T of approximately 91milliseconds) for example, and whether the mean frequency is less than apredetermined lower mean frequency, such as 3 Hz for example. If themean frequency is below a second, lower threshold, such as 3 Hz, forexample, the signal is also rejected as noise and no further noise testsare initiated. This comparison of the mean frequency to a second lowerthreshold is intended to identify instances of oversensing, resulting inappropriate transition to the concerned state. If the mean frequency ofthe signal is less than 3 Hz, it is generally not possible for the heartrate to be greater than 180 beats per minute. In practice, it may beadvantageous to set the lower frequency threshold equal to theprogrammed VT/VF detection rate, which is typically approximately 3 Hz.

Therefore, in the determination of Block 388, if the mean frequency isdetermined to be either greater than or equal to the predetermined uppermean frequency threshold or less than the lower threshold, the threesecond segment for that channel is identified as being likely corruptedwith noise, Block 386. If the mean frequency is determined to be bothless than the predetermined upper mean frequency threshold and greaterthan the lower threshold, the three second segment for that channel isidentified as not being noise corrupted, Block 390.

Returning to FIG. 4 , once the determination as to whether the channelsECG1 and ECG2 are corrupted by noise is made, Block 342, a determinationis made as to whether both channels are determined to be noisecorrupted, Block 344. If the signal associated with both channels ECG1and ECG2 is determined to likely be corrupted by noise, both channelsare classified as being not shockable, Block 347, and therefore a bufferfor each channel ECG1 and ECG 2 containing the last threeclassifications of the channel is updated accordingly and the process isrepeated for the next three-second windows. If both channels ECG1 andECG2 are not determined to be likely corrupted by noise, No in Block344, the device distinguishes between either one of the channels beingnot corrupted by noise or both channels being not corrupted by noise bydetermining whether noise was determined to be likely in only one of thetwo channels ECG1 and ECG2, Block 346.

If noise was likely in only one of the two channels, a determination ismade whether the signal for the channel not corrupted by noise, i.e.,the clean channel, is more likely associated with a VT event or with aVF event by determining, for example, whether the signal for thatchannel includes R-R intervals that are regular and the channel can betherefore classified as being relatively stable, Block 348. If the R-Rintervals are determined not to be relatively stable, NO in Block 348,the signal for that channel is identified as likely being associatedwith VF, which is then verified by determining whether the signal is ina VF shock zone, Block 350, described below. If R-R intervals for thatchannel are determined to be stable, YES in Block 348, the signal isidentified as likely being associated with VT, which is then verified bydetermining whether the signal is in a VT shock zone, Block 352,described below.

If noise was not likely for both of the channels, No in Block 346, i.e.,both channels are determined to be clean channels, a determination ismade whether the signal for both channels is more likely associated witha VT event or with a VF event by determining whether the signal for bothchannels includes R-R intervals that are regular and can be thereforeclassified as being relatively stable, Block 356. The determination inBlock 356 of whether the R-R intervals are determined to be relativelystable may be made using the method described in U.S. Pat. No. 7,894,894to Stadler et al., incorporated herein by reference in it's entirety. Ifthe R-R intervals are determined not to be relatively stable, NO inBlock 356, the signal for both channels is identified as likely beingassociated with VF, which is then verified by determining whether thesignal for each channel is in a VF shock zone, Block 360, describedbelow. If R-R intervals for both channels are determined to be stable,YES in Block 356, the signal is identified as likely being associatedwith VT, which is then verified by determining, based on both channels,whether the signal is in a VT shock zone, Block 358.

FIG. 7 is a graphical representation of a VF shock zone according to anembodiment of the present invention. As illustrated in FIG. 7 , a VFshock zone 500 is defined for each channel ECG1 and ECG2 based on therelationship between the calculated low slope content and the spectralwidth associated with the channel. For example, the shock zone isdefined by a first boundary 502 associated with the low slope contentset for by the equation:Low slope content=−0.0013×spectral width+0.415  Equation 1and a second boundary 504 associated with the spectral width set forthby the equation:spectral width=200  Equation 2

The low slope content metric is calculated as the ratio of the number ofdata points with low slope to the total number of samples in the3-second segment. For example, according to an embodiment of the presentinvention, the difference between successive ECG samples is determinedas an approximation of the first derivative (i.e, the slope) of the ECGsignal. In particular, the raw signal for each channel is applied to afirst order derivative filter to obtain a derivative signal for thethree-second segment. The derivative signal is then rectified, dividedinto four equal sub-segments, and the largest absolute slope isestimated for each of the four sub-segments.

A determination is made as to whether the largest absolute slopes areless than a portion of the overall largest absolute slope for the wholethree-second segment, such as one-fifth of the overall absolute slope,for example. If the largest absolute slope is less than the portion ofthe overall slope, then the slope value for that sub-segment is setequal to the overall largest absolute slope. If the largest absoluteslope is not less than the portion of the overall slope, then the slopevalue for that sub-segment is set equal to the determined largestabsolute slope for the sub-segment.

Once the slope value for each of the sub-segments has been determinedand updated by being set equal to the largest slope for the three secondsegment, if necessary, the average of the four slopes is calculated anddivided by a predetermined factor, such as 16 for example, to obtain alow slope threshold. The low slope content is then obtained bydetermining the number of sample points in the three-second segmenthaving an absolute slope less than or equal to the low slope threshold.

According to an embodiment of the present invention, if, during thedetermination of the low slope threshold, the low slope threshold is afraction, rather than a whole number, a correction is made to the lowslope content to add a corresponding fraction of the samples. Forexample, if the threshold is determined to be 4.5, then the low slopecontent is the number of sample points having an absolute slope lessthan or equal to 4 plus one half of the number of sample points withslope equal to 5.

The spectral width metric, which corresponds to an estimate of thespectral width of the signal for the three-second segment associatedwith each channel ECG1 and ECG2, is defined, for example, as thedifference between the mean frequency and the fundamental frequency ofthe signal. According to an embodiment of the present invention, thespectral width metric is calculated by determining the differencebetween the most recent estimate of the RR-cycle length and the meanspectral period of the signal for that channel. As is known in the art,the mean spectral period is the inverse of the mean spectral frequency.

As can be seen in FIG. 7 , since noise 506 tends to have a relativelyhigher spectral width, and normal sinus rhythm 508 tends to have arelatively higher low slope content relative to VF, both noise 506 andnormal sinus rhythm 508 would be located outside the VF shock zone 500.

A determination is made for each channel ECG1 and ECG2 as to whether thelow slope content for that channel is less than both the first boundary502 and the spectral width is less than the second boundary 504, i.e.,the low slope content is less than −0.0013×spectral width+0.415, and thespectral width is less than 200. For example, once the event isdetermined to be associated with VF, i.e., the intervals for bothchannels are determined to be irregular, No in Block 356, adetermination is made that channel ECG1 is in the VF shock zone, Yes inBlock 360, if, for channel ECG1, both the low slope content is less thanthe first boundary 502 and the spectral width is less than the secondboundary 504. The three second segment for that channel ECG1 is thendetermined to be shockable, Block 363 and the associated buffer for thatchannel is updated accordingly. If either the low slope content for thechannel is not less than the first boundary 502 or the spectral width isnot less than the second boundary, the channel ECG1 is determined not tobe in the VF shock zone, No in Block 360, the three second segment forthat channel ECG1 is then determined to be not shockable, Block 365, andthe associated buffer is updated accordingly.

Similarly, a determination is made that channel ECG2 is in the VF shockzone, Yes in Block 362, if, for channel ECG2, both the low slope contentis less than the first boundary 502 and the spectral width is less thanthe second boundary 504. The three second segment for that channel ECG2is then determined to be shockable, Block 369 and the associated bufferfor that channel is updated accordingly. If either the low slope contentfor the channel is not less than the first boundary 502 or the spectralwidth is not less than the second boundary, the channel ECG2 isdetermined not to be in the VF shock zone, No in Block 362, the threesecond segment for that channel ECG2 is then determined to be notshockable, Block 367, and the associated buffer is updated accordingly.

FIGS. 8A and 8B are graphical representations of the determination ofwhether an event is within a shock zone according to an embodiment ofthe present invention. During the determination of whether the event iswithin the VT shock zone, Block 358 of FIG. 4 , the low slope contentand the spectral width is determined for each channel ECG1 and ECG2, asdescribed above in reference to determining the VF shock zone. Adetermination is made as to which channel of the two signal channelsECG1 and ECG2 contains the minimum low slope content and which channelof the two signal channels ECG 1 and ECG2 contains the minimum spectralwidth. A first VT shock zone 520 is defined based on the relationshipbetween the low slope content associated with the channel determined tohave the minimum low slope content and the spectral width associatedwith the channel determined to have the minimum spectral width. Forexample, according to an embodiment of the present invention, the firstVT shock zone 520 is defined by a boundary 522 associated with theminimum low slope content and the minimum spectral width set forth bythe equation:LSC=−0.004×SW+0.93  Equation 3

A second VT shock zone 524 is defined based on the relationship betweenthe low slope content associated with the channel determined to have theminimum low slope content and the normalized mean rectified amplitudeassociated with the channel determined to have the maximum normalizedmean rectified amplitude. In order to determine the normalized meanrectified amplitudes for the two channels ECG1 and ECG2 utilized duringthe VT shock zone test, the amplitude of each sample associated with thethree second window is determined, resulting in N sample amplitudes,from which a mean rectified amplitude is calculated as the ratio of thesum of the rectified sample amplitudes to the total number of sampleamplitudes N for the segment. If the sampling rate is 256 samples persecond, for example, the total number of sample amplitudes N for thethree-second segment would be N=768 samples.

According to an embodiment of the present invention, for example, thesecond VT shock zone 524 is defined by a second boundary 526 associatedwith the relationship between the minimum low slope count and themaximum normalized mean rectified amplitude set forth by the equation:NMRA=68×LSC+8.16  Equation 4

If both the minimum low slope count is less than the first boundary 522,i.e., −0.004×minimum spectral width+0.93, and the maximum normalizedmean rectified amplitude is greater than the second boundary 526, i.e.,68×minimum low slope count+8.16, the event is determined to be in the VTshock zone, YES in Block 358, and both channels ECG1 and ECG2 aredetermined to be shockable, Block 357, and the associated buffers areupdated accordingly. If either the minimum low slope count is not lessthan the first boundary 522 or the maximum normalized mean rectifiedamplitude is not greater than the second boundary 526, the event isdetermined to be outside the VT shock zone, NO in Block 358, and bothchannels ECG1 and ECG2 are determined to be not shockable, Block 359.

As described, during both the VF shock zone test, Blocks 360 and 362,and the VT shock zone test, Block 358, the test results for each channelECG1 and ECG2 as being classified as shockable or not shockable arestored in a rolling buffer containing the most recent eight suchdesignations, for example, for each of the two channels ECG1 and ECG2that is utilized in the determination of Block 356, as described below.

If only one of the two channels ECG1 and ECG2 is determined to becorrupted by noise, Yes in Block 346, a determination is made whetherthe signal for the channel not corrupted by noise, i.e., the “cleanchannel”, is more likely associated with a VT event or with a VF eventby determining whether the signal for the clean channel includes R-Rintervals that are regular and can be therefore classified as beingrelatively stable, Block 348. If the R-R intervals are determined not tobe relatively stable, NO in Block 348, the signal for the clean channelis identified as likely being associated with VF, which is then verifiedby determining whether the signal for the clean channel is in a VF shockzone, Block 350, described below. If R-R intervals for the clean channelare determined to be stable, YES in Block 348, the signal is identifiedas likely being associated with VT, which is then verified bydetermining whether the signal for the clean channel is in a VT shockzone, Block 352.

According to an embodiment of the present invention, in order todetermine whether the signal for the clean channel includes R-Rintervals that are regular and the clean channel can be thereforeclassified as being either relatively stable, Yes in Block 348, orrelatively unstable, No in Block 348, the device discriminates VT eventsfrom VF events in Block 348 by determining whether the relative level ofvariation in the RR-intervals associated with the clean channel isregular. FIG. 9 is a flowchart of a method for discriminating cardiacevents according to an embodiment of the disclosure. For example, asillustrated in FIG. 9 , predetermined maximum and minimum intervals forthe clean channel are identified from the updated buffer of 12RR-intervals, Block 342 of FIG. 4 . According to an embodiment of thepresent invention, the largest RR-interval and the sixth largestRR-interval of the twelve RR-intervals are utilized as the maximuminterval and the minimum interval, respectively.

The difference between the maximum RR-interval and the minimumRR-interval of the 12 RR-intervals is calculated to generate an intervaldifference associated with the clean channel, 702. A determination isthen made as to whether the interval difference is greater than apredetermined stability threshold, Block 704, such as 110 milliseconds,for example.

If the interval difference is greater than the stability threshold, theevent is classified as an unstable event, Block 706, and therefore theclean channel is determined not to include regular intervals, No inBlock 348, and a determination is made as to whether the signalassociated with the clean channel is within a predetermined VF shockzone, Block 350 of FIG. 4 , described below. If the interval differenceis less than or equal to the stability threshold, No in Block 704, thedevice determines whether the minimum RR interval is greater than aminimum interval threshold, Block 710, such as 200 milliseconds, forexample.

If the minimum interval is less than or equal to the minimum intervalthreshold, No in Block 710, the event is classified as an unstableevent, Block 706, and therefore the clean channel is determined not toinclude regular intervals, No in Block 348, and a determination is madeas to whether the signal associated with the clean channel is within apredetermined VF shock zone, Block 350 of FIG. 4 , described below. Ifthe minimum interval is greater than the minimum interval threshold, Yesin Block 710, the device determines whether the maximum interval is lessthan or equal to a maximum interval threshold, Block 712, such as 333milliseconds for example. If the maximum interval is greater than themaximum interval threshold, the event is classified as an unstableevent, Block 706, and therefore the clean channel is determined not toinclude regular intervals, No in Block 348, and a determination is madeas to whether the signal associated with the clean channel is within apredetermined VF shock zone, Block 350 of FIG. 4 , described below. Ifthe maximum interval is less than or equal to the maximum intervalthreshold, the event is classified as a stable event, Block 714, andtherefore the clean channel is determined to include regular intervals,Yes in Block 348, and a determination is made as to whether the signalassociated with the clean channel is within a predetermined VT shockzone, Block 352 of FIG. 4 , described below.

Returning to FIG. 4 , the determination of whether the clean channel iswithin the VF shock zone, Block 350, is made based upon a low slopecontent metric and a spectral width metric, similar to the VF shock zonedetermination described above in reference to Blocks 360 and 362, bothof which are determined for the clean channel using the method describedabove. Once the low slope content metric and a spectral width metric aredetermined for the clean channel, the determination of whether the cleanchannel is in the VF shock zone is made using Equations 1 and 2, so thatif either the low slope content for the clean channel is not less thanthe first boundary 502 or the spectral width is not less than the secondboundary 504, the clean channel is determined not to be in the VF zone,No in Block 350 and both channels are classified as not shockable, Block351, and the associated buffers are updated accordingly.

If the low slope content for the clean channel is less than the firstboundary 502 and the spectral width is less than the second boundary504, the clean channel is determined to be in the VF zone, Yes in Block350. A determination is then made as to whether the channel determinedto be corrupted by noise, i.e., the “noisy channel”, is within the VFshock zone, Block 354. If either the low slope content for the noisychannel is not less than the first boundary 502 or the spectral width isnot less than the second boundary 504, the noisy channel is determinednot to be in the VF zone, No in Block 354, the clean channel isclassified as shockable and the noisy channel is classified as notshockable, Block 355, and the associated buffers are updatedaccordingly.

If the low slope content for the noisy channel is less than the firstboundary 502 and the spectral width is less than the second boundary504, the noisy channel is determined to be in the VF zone, Yes in Block354, both the clean channel and the noisy channel are classified asbeing shockable, Block 353, and the associated buffers are updatedaccordingly.

Similar to the VT shock zone determination described above in referenceto Block 358, during the determination as to whether the clean channelis within the VT shock zone in Block 352, the low slope content and thespectral width is determined for the clean channel as described above inreference to determining the VF shock zone. The first VT shock zone 520is defined based on the relationship between the low slope content andthe spectral width associated with the clean channel according toEquation 3, for example, and the second VT shock zone 524 is definedbased on the relationship between the low slope count and the normalizedmean rectified amplitude associated with the clean channel. Thenormalized mean rectified amplitudes for the clean channel is the sameas described above in reference to the noise detection tests of Block344. For example, according to an embodiment of the present invention,the second VT shock zone 524 is defined by a second boundary 526associated with the relationship between the low slope count and thenormalized mean rectified amplitude of the clean channel using Equation4.

If both the low slope count is less than the first boundary 522, i.e.,−0.004×spectral width of clean channel+0.93, and the normalized meanrectified amplitude is greater than the second boundary 526, i.e.,68×low slope count of clean channel+8.16, the clean channel isdetermined to be in the VT shock zone, Yes in Block 352, both channelsare classified as being shockable, Block 353, and the associated buffersare updated accordingly.

If either the low slope count is not less than the first boundary 522 orthe maximum normalized mean rectified amplitude is not greater than thesecond boundary 526, the clean channel is determined to be outside theVT shock zone, No in Block 352, both channels are classified as beingnot shockable, Block 351, and the associated buffers are updatedaccordingly.

According to an embodiment of the present disclosure, in addition to theclassification of the sensing channels ECG1 and ECG2 as being shockableor not shockable using a gross morphology analysis, as described in FIG.4 , for example, the device also performs a beat-based analysis of thebeats within each of the three-second windows, Block 368, so that thedecision on state transitions (e.g. as to whether to transition from theconcerned operating state 304 to the armed operating state 306 in Block370, or from the armed state 306 to the shock state 308) is made basedon the results of both an analysis of the gross morphology of the signalin the three-second window or windows for each sensing channel ECG1 andECG2, and an analysis of the morphology of individual beats or R-wavesin the three-second window or windows for each sensing channel ECG1 andECG2, as described below. For a three-second segment to be classified asshockable, both the gross morphology and beat-based analysis have toclassify the same three-second segment as shockable.

For example, according to an embodiment of the present invention, inorder to determine whether to transition from the concerned operatingstate 304 to the armed operating state 306, the device determineswhether a predetermined number, such as two out of three for example, ofthree-second segments for both channels ECG1 and ECG2 have beenclassified as being shockable during the gross morphology analysis,Blocks 353, 357, 363 and 369, and determines whether those three-secondsegments for both channels have also been classified as being shockableduring the beat-based analysis, Block 368. If the predetermined numberof three-second segments in both channels ECG1 and ECG2 have beenclassified as shockable during both the gross morphology analysis andthe beat-based analysis, the device transitions from the concerned state304 to the armed state 306, Yes in Block 370. When the device determinesto transition from the concerned state 304 to the armed state 306, Yesin Block 370, processing continues to be triggered by a three-secondtime out as is utilized during the concerned state 304, described above.

If the predetermined number of three-second segments in both channelsECG1 and ECG2 have not been classified as shockable during both thegross morphology analysis and the beat-based analysis, the device doesnot transition from the concerned state 304 to the armed state 306, Noin Block 370, and a determination as to whether to transition back tothe not concerned state 302 is made, Block 372. The determination as towhether to transition from the concerned state 304 back to the notconcerned state 302 is made, for example, by determining whether a heartrate estimate is less than a heart rate threshold level in both of thetwo channels ECG1 and ECG2, using the method for determining a heartrate estimate as described in U.S. Pat. No. 7,894,894 to Stadler et al.,incorporated herein by reference in it's entirety. If it is determinedthat the device should not transition to the not concerned state 302,i.e., either of the two heart rate estimates are greater than the heartrate threshold, No in Block 372, the process continues using the signalgenerated during a next three-second window, Block 341.

As described above, the determination of whether the sensing channelsECG1 and ECG2 are shockable or not shockable, Blocks 353, 355, 357, and363-369, is performed by analyzing the gross morphology of a sensedwaveform occurring within the three-second windows. The ECG signal issegmented into n-second intervals, i.e., 3 second intervals, that areused for determining gross morphology features of the three-secondwaveform. In particular, the gross morphology features are determinedacross an n-second time interval without relying on R-wave sensing andare therefore features making up the whole waveform signal that can bedetermined from the ECG signal independent of individual cardiac signalsof the cardiac cycle, i.e., individual beats or R-waves contained withinthe three-second window that are within the entire three-second window.A single waveform in the n-second window begins at the start of thewindow, extends through entire window, ending at the end of thethree-second window so that a single morphology determination is madefor the single waveform included within the single three-second window.

On the other hand, multiple cardiac cycles, i.e, R-waves signals, areincluded within the three-second window, and therefore the n-secondwindow may start and end at any time point relative to each of theindividual R-wave signals irrespective of where an individual R-wavesignal starts and ends, so that multiple individual beat-baseddeterminations are made for the multiple beat waveforms included withinthe single three-second window.

Morphology features computed for the single waveform extending acrossthe n-second time period are referred to as “gross” morphology featuresbecause the features are characteristics of the single signal, extendingfrom the start to the end of the window, that is extracted, independentof cardiac cycle timing, from a time segment that includes multipleindividual cardiac cycles. In contrast, morphology features extractedfrom the ECG signal during a cardiac cycle are referred to as“beat-based” features. Beat-based features are determined from an ECGsignal segment over a time interval of one cardiac cycle of multiplecardiac cycles contained within a single three-second window. Beat-basedfeatures may be averaged or determined from multiple cardiac cycles butare representative of a single feature of the ECG signal during acardiac cycle. Determination of a beat feature is dependent onidentifying the timing of a cardiac cycle, or at least a sensed eventsuch as an R-wave, as opposed to determining gross features independentof the cardiac cycle over a time segment that is typically longer thanone cardiac cycle.

FIG. 10 is a flowchart of a beat-based analysis during detection ofarrhythmias in a medical device according to an embodiment of thepresent disclosure. Therefore, as described above, in addition toperforming the morphology analysis of the whole waveform within thethree-second windows associated with each sensing channel ECG1 and ECG2,the device performs a beat-based analysis of the signal sensedsimultaneously within both channels ECG1 and ECG2, Block 368. Inparticular, as illustrated in FIG. 10 , for each three-second sensingwindow associated with the respective sensing channels ECG1 and ECG2,the device locates a beat, i.e., R-wave, Block 800, and compares theindividual beat to a predetermined beat template, Block 802, such as anormal sinus rhythm template, for example. Based upon the comparison ofthe beat to the template, the device determines whether the beat iseither a match beat or a non-match beat by determining the extent towhich the beat matches the template, Block 804. For example, in order toidentify the beat as either a match beat or a non-match beat, the devicedetermines in Block 804 whether the beat matches the sinus rhythmtemplate within a predetermined percentage, such as 60 percent, forexample. If the beat matches the template by the predeterminedpercentage or greater, Yes in Block 804, the beat is identified as amatch beat and the number of match beats for the three-second window isupdated, Block 806. If the beat matches the template by less than thepredetermined percentage, No in Block 804, the beat is identified as anon-match beat and the number of non-match beats for the three-secondwindow is updated, Block 808.

Once the beat is identified as likely being either a match beat or anon-match beat, the device determines whether the match/non-matchdetermination has been made for all of the beats in the three-secondwindow, Block 810. If the determination has not been made for all of thebeats in the three-second window, No in Block 810, the process isrepeated with another beat located within the three-second window. Oncethe determination has been made for all of the beats in the three-secondwindow, Yes in Block 810, a determination is made as to whether thenumber of non-match beats in the three-second window is greater than anon-match threshold, Block 812. According to an embodiment of thedisclosure, the non-match threshold is set as a predeterminedpercentage, such as 75 percent for example, so that if the number ofindividual beats in the three-second window that are identified as beingnon-match beats is greater than 75 percent of the number of all of thebeats in the window, Yes in Block 812, the three-second window isidentified as being shockable based on beat-based analysis, Block 814.On the other hand, if the number of individual beats in the three-secondwindow that are identified as being non-match beats is not greater than75 percent of the number of all of the beats in the window, No in Block812, the three-second window is identified as being not shockable basedon beat based analysis, Block 814. The beat-based analysis determinationof the three-second windows as being shockable 814 or not shockable,Block 816 is then used in combination with the waveform morphologyanalysis of both of the three-second windows being shockable, Blocks353, 357, 363 and 369 or both not shockable, Blocks 351, 355, 359, 365and 367 to determine whether to transition to the next state, Block 370,as described above.

As can be seen in FIG. 4 , the way in which both channels ECG1 and ECG2could have been determined to be shockable can vary. First, if noise wasnot determined to be occurring in either channel, No in Block 346, butboth channels are determined to have regular intervals, Yes in Block356, and both channels are determined to be in the VT shock zone, Yes inBlock 358, both of the sensing channels ECG1 and ECG2 are determined tobe shockable, Block 359. Second, if noise was not determined to beoccurring in either channel, No in Block 346, but both channels are notdetermined to have regular intervals, No in Block 356, and both channelsare determined to be in the VF shock zone, Yes in Blocks 360 and 362,both of the sensing channels ECG1 and ECG2 are determined to beshockable.

However, if noise was determined to be occurring in one channel, Yes inBlock 346, but the clean channel was determined to have regularintervals, Yes in Block 348, and to be in the VT shock zone, Yes inBlock 352, both of the sensing channels ECG1 and ECG2 are determined tobe shockable, Block 353. Finally, if noise was determined to beoccurring in one channel, Yes in Block 346, the clean channel wasdetermined not to have regular intervals, No in Block 348, and both theclean and the noisy channel are determined to be in the VF shock zone,Yes in Blocks 350 and 354, both of the sensing channels ECG1 and ECG2are determined to be shockable, Block 353.

In this way, both channels may be determined to be shockable based on adetermination that both channels are either in the VF shock zone, Blocks363 and 369, or Block 353 via Blocks 350 and 354, based on adetermination that both channels are in the VT shock zone, Block 357, orbased on a determination that only one channel, i.e., the clean channel,is within the VT shock zone, Block 353 via Block 352.

FIG. 11 is a flowchart of a beat-based analysis during detection ofarrhythmias in a medical device according to an embodiment of thepresent disclosure. Therefore, according to an embodiment of the presentdisclosure, the device may initially identify how the three-secondwindows were determined to be shockable during the gross morphologyanalysis, i.e., by using both channels or only one channel, and based onthis determination, determine which channels that are to be utilized inthe beat morphology analysis.

Therefore, as illustrated in FIGS. 4 and 11 , according to oneembodiment, the device determines whether both channels were used in theidentification of both channels being shockable, Block 820, so that ifboth channels were utilized, Yes in Block 820, the beat-based analysis,Block 368, is performed for both channels, Block 822, as described abovein FIG. 10 .

If both channels were not utilized, No in Block 820, the beat-basedanalysis, Block 368, is performed for only one channel, i.e., the cleanchannel, Block 824. In particular, the device locates a beat, i.e.,R-wave, in only the clean channel, Block 800, and compares theindividual beat to a predetermined beat template, Block 802, such as anormal sinus rhythm template, for example. Based upon the comparison ofthe beat to the template, the device determines whether the beat iseither a match beat or a non-match beat by determining the extent towhich the beat matches the template, Block 804. For example, in order toidentify the beat as either a match beat or a non-match beat, the devicedetermines in Block 804 whether the beat matches the sinus rhythmtemplate within a predetermined percentage, such as 60 percent, forexample. If the beat matches the template by the predeterminedpercentage or greater, Yes in Block 804, the beat is identified as amatch beat and the number of match beats for the three-second window isupdated, Block 806. If the beat matches the template by less than thepredetermined percentage, No in Block 804, the beat is identified as anon-match beat and the number of non-match beats for the three-secondwindow is updated, Block 808.

Once the beat is identified as likely being either a match beat or anon-match beat, the device determines whether the match/non-matchdetermination has been made for all of the beats in the three-secondwindow of only the clean channel, Block 810. If the determination hasnot been made for all of the beats in the three-second window for theclean channel, No in Block 810, the process is repeated with anotherbeat located within the three-second window of the clean channel. Oncethe determination has been made for all of the beats in the three-secondwindow of the clean channel, Yes in Block 810, a determination is madeas to whether the number of non-match beats in the three-second windowis greater than a non-match threshold, Block 812. According to anembodiment of the disclosure, the non-match threshold is set as apredetermined percentage, such as 75 percent for example, so that if thenumber of individual beats in the three-second window that areidentified as being non-match beats is greater than 75 percent of thenumber of all of the beats in the window, Yes in Block 812, thethree-second window of the clean channel is identified as beingshockable based on the beat-based analysis, Block 814. On the otherhand, if the number of individual beats in the three-second window thatare identified as being non-match beats is not greater than 75 percentof all of the number of the beats in the window, No in Block 812, thethree-second window of the clean channel is identified as being notshockable based on the beat-based analysis, Block 814.

The decision as to whether to transition from the concerned operatingstate 304 to the armed operating state 306 in Block 370 is made based onthe results of both an analysis of the morphology of the signal in thethree-second window or windows for each sensing channel ECG1 and ECG2,and an analysis of morphology of individual beats or R-waves in thethree-second window or windows for each sensing channel ECG1 and ECG2,as described above. In the instance where the beat-based analysis wasperformed for only one channel, i.e., the clean channel, Block 824, thedetermination of whether to transition to the next state, Block 370,would be satisfied if both the predetermined number of three-secondsegments in both channels ECG1 and ECG2 have been classified asshockable during the gross morphology analysis, and the beat-basedanalysis, Block 368, is satisfied for only the clean channel, andtherefore the device transitions from the concerned state 304 to thearmed state 306, Yes in Block 370. If the predetermined number ofthree-second segments in both channels ECG1 and ECG2 have not beenclassified as shockable during both the gross morphology analysis andthe beat-based analysis of only the clean channel, the device does nottransition from the concerned state 304 to the armed state 306, No inBlock 370, and a determination as to whether to transition back to thenot concerned state 302 is made, Block 372, as described above.

Thus, a method and apparatus for discriminating a cardiac event havebeen presented in the foregoing description with reference to specificembodiments. It is appreciated that various modifications to thereferenced embodiments may be made without departing from the scope ofthe disclosure as set forth in the following claims.

I claim:
 1. A device comprising: circuitry configured to obtain a firstcardiac signal; a processor configured to: identify a plurality ofanalysis windows, each of the plurality of analysis windows extending apredetermined amount of time and including a respective segment of thefirst cardiac signal having a plurality of R-wave sense events; perform,for each of the plurality of analysis windows, a first analysis of therespective segment of the first cardiac signal across the entireanalysis window; perform, for each of the plurality of analysis windows,a second analysis of each individual one of the plurality of R-wavesense events of the respective segment of the first cardiac signalwithin the analysis window; independently classify, for each of theplurality of analysis windows, the first analysis and the secondanalysis as shockable or not shockable; classify analysis windows of theplurality of analysis windows as shockable when both the first analysisresults in a shockable classification and the second analysis results ina shockable classification; and detect a tachyarrhythmia when athreshold number of the plurality of analysis windows are classified asshockable; and one or more capacitors configured to be charged inresponse to detecting the tachyarrhythmia and discharged to deliver anelectrical therapy to treat the tachyarrhythmia.
 2. The device of claim1, wherein the processor is configured to, during the second analysis ofeach of the plurality of R-waves within a respective analysis window:compare a beat morphology of the respective R-wave sense event to a beattemplate; classify the respective R-wave sense event as a non-match beatwhen the respective comparison does not meet a match criteria; andclassify the respective analysis window as shockable when the number ofR-wave sense events classified as non-match beats is greater than anon-match beat threshold.
 3. The device of claim 2, wherein thenon-match beat threshold is equal to 75 percent of the number of R-wavesense events in the analysis window.
 4. The device of claim 1, whereinthe processor is configured to determine whether the respective segmentof the cardiac signal in the analysis window is noisy during the firstanalysis and classify the respective segment as shockable or notshockable based on at least the determination.
 5. The device of claim 1,wherein the processor is configured to determine a plurality of R-Rintervals between consecutive R-wave sense events during the firstanalysis of the respective segment of the first cardiac signal,determine whether the plurality of determined R-R intervals are stable,analyze a first set of morphology parameters across the entire analysiswindow when the R-R intervals are determined to be stable, and analyze asecond set of morphology parameters across the entire analysis windowwhen the R-R intervals are determined to be unstable.
 6. The device ofclaim 5, wherein the processor is further configured to determine aminimum R-R interval of the plurality of determined R-R intervals of therespective segment, determine a maximum R-R interval of the plurality ofdetermined R-R intervals of the respective segment, and determinewhether the plurality of determined R-R intervals are stable based atleast on the minimum R-R interval and the maximum R-R interval.
 7. Thedevice of claim 5, wherein the processor is configured to determine theplurality of determined R-R intervals are unstable when at least one ofa difference between the maximum R-R interval and the minimum R-Rinterval is greater than a stability threshold, the minimum R-R intervalis less than or equal to a minimum interval threshold or the maximum R-Rinterval is greater than a maximum interval threshold.
 8. The device ofclaim 7, wherein the processor is configured to determine the pluralityof determined R-R intervals are stable when a difference between themaximum R-R interval and the minimum R-R interval is less than or equalto the stability threshold, the minimum R-R interval is greater than theminimum interval threshold, and the maximum R-R interval is less than orequal to the maximum interval threshold.
 9. The device of claim 1,wherein the first analysis and the second analysis occur in response todetecting short R-R intervals between consecutive R-wave sense events.10. The device of claim 1, wherein the first cardiac signal is sensed ona first sensing vector formed between a first electrode and a secondelectrode, the device further comprising second circuitry configured toobtain a second cardiac signal sensed on a second sensing vector formedbetween a third electrode and a fourth electrode, at least one of thethird electrode and the fourth electrode being different than the firstelectrode and the second electrode, the processor further configured to:identify a second plurality of analysis windows, each of the secondplurality of analysis windows extending the predetermined amount of timeand including a respective segment of the second cardiac signal having aplurality of R-wave sense events; perform, for each of the secondplurality of analysis windows, the first analysis of the respectivesegment of the second cardiac signal across the entire analysis window;perform, for each of the second plurality of analysis windows, thesecond analysis of each individual one of the plurality of R-wave senseevents of the respective segment of the second cardiac signal within theanalysis window; classify analysis windows of the second plurality ofanalysis windows as shockable when both the first analysis results in ashockable classification and the second analysis results in a shockableclassification; and detect the tachyarrhythmia when the threshold numberof the first plurality of analysis windows are classified as shockableand a second threshold number of the second plurality of analysiswindows are classified as shockable.
 11. The device of claim 10, whereinthe processor is configured to determine that no tachyarrhythmia isdetected either when the threshold number of the first plurality ofanalysis windows are not classified as shockable or the second thresholdnumber of the second plurality of analysis windows are not classified asshockable.
 12. The device of claim 10, wherein the first analysis of thesegments of the first and second cardiac signal includes determining oneor more of the segments within the first plurality of analysis windowsof the first cardiac signal are noisy, the processor being furtherconfigured to: perform one or both of the first analysis of the firstcardiac signal and the second analysis of the first cardiac signalwithout the respective ones of the first plurality of analysis windowsof the first cardiac signal determined to be noisy.
 13. A methodcomprising: obtaining a first cardiac signal; identifying a plurality ofanalysis windows, each of the plurality of analysis windows extending apredetermined amount of time and including a respective segment of thefirst cardiac signal having a plurality of R-wave sense events;performing, for each of the plurality of analysis windows, a firstanalysis of the respective segment of the first cardiac signal acrossthe entire analysis window; performing, for each of the plurality ofanalysis windows, a second analysis of each individual one of theplurality of R-wave sense events of the respective segment of the firstcardiac signal within the analysis window; independently classifying,for each of the plurality of analysis windows, the first analysis andthe second analysis as shockable or not shockable; classifying analysiswindows of the plurality of analysis windows as shockable when both thefirst analysis results in a shockable classification and the secondanalysis results in a shockable classification; detecting atachyarrhythmia when a threshold number of the plurality of analysiswindows are classified as shockable; charging one or more capacitors inresponse to detecting the tachyarrhythmia; and discharging the one ormore capacitors to deliver an electrical therapy to treat thetachyarrhythmia.
 14. The method of claim 13, wherein performing, foreach of the plurality of analysis windows, the second analysiscomprises: comparing a beat morphology of the respective R-wave senseevent to a beat template; classifying the respective R-wave sense eventas a non-match beat when the respective comparison does not meet a matchcriteria; and classifying the respective analysis window as shockablewhen the number of R-wave sense events classified as non-match beats isgreater than a threshold.
 15. The method of claim 13, furthercomprising: determining a plurality of R-R intervals between consecutiveR-wave sense events during the first analysis of the respective segmentof the first cardiac signal; determining whether the plurality ofdetermined R-R intervals are stable; analyzing a first set of morphologyparameters across the entire analysis window when the R-R intervals aredetermined to be stable; and analyzing a second set of morphologyparameters across the entire analysis window when the R-R intervals aredetermined to be unstable.
 16. The method of claim 13, furthercomprising: determining a minimum R-R interval of the plurality ofdetermined R-R intervals of the respective segment; determining amaximum R-R interval of the plurality of determined R-R intervals of therespective segment; and determining whether the plurality of determinedR-R intervals are stable based at least on the minimum R-R interval andthe maximum R-R interval.
 17. The method of claim 13, further comprisingdetecting short R-R intervals between consecutive R-wave sense events,wherein the first analysis and the second analysis occur in response todetecting short R-R intervals between consecutive R-wave sense events.18. The method of claim 13, wherein the cardiac signal comprises a firstcardiac signal sensed on a first sensing vector formed between a firstelectrode and a second electrode, the method further comprising:obtaining a second cardiac signal sensed on a second sensing vectorformed between a third electrode and a fourth electrode, at least one ofthe third electrode and the fourth electrode being different than thefirst electrode and the second electrode; identifying a second pluralityof analysis windows, each of the second plurality of analysis windowsextending the predetermined amount of time and including a respectivesegment of the second cardiac signal having a plurality of R-wave senseevents; performing, for each of the second plurality of analysiswindows, the first analysis of the respective segment of the secondcardiac signal across the entire analysis window; performing, for eachof the second plurality of analysis windows, the second analysis of eachindividual one of the plurality of R-wave sense events of the respectivesegment of the second cardiac signal within the analysis window;classifying analysis windows of the second plurality of analysis windowsas shockable when both the first analysis results in a shockableclassification and the second analysis results in a shockableclassification; and wherein detecting the tachyarrhythmia comprisesdetecting the tachyarrhythmia when the threshold number of the firstplurality of analysis windows are classified as shockable and a secondthreshold number of the second plurality of analysis windows areclassified as shockable.
 19. The method of claim 18, further comprisingdetermining that no tachyarrhythmia is detected either when thethreshold number of the first plurality of analysis windows are notclassified as shockable or the second threshold number of the secondplurality of analysis windows are not classified as shockable.
 20. Themethod of claim 18, further comprising: determining, during the firstanalysis of the segments of the first and second cardiac signal, one ormore of the segments within the first plurality of analysis windows ofthe first cardiac signal are noisy; and performing one or both of thefirst analysis of the first cardiac signal and the second analysis ofthe first cardiac signal without the respective ones of the firstplurality of analysis windows of the first cardiac signal determined tobe noisy.